Positive-displacement liquid-pressure machines and pressure-balanced journal/thrust bushes therefor

ABSTRACT

A positive-displacement machine having meshing rotors is provided with an end plate in the form of a journal/thrust bush, associated with a rotor, which has a groove forming a speed slot associated with its face adjacent to the rotor. The end plate is so subjected to liquid pressures on defined areas of its end face remote from the rotor as to be urged into adequate sealing engagement with the rotor. A zone of the said face of the end plate adjacent to the rotor which is subjected, during operation of the machine, to fluctuating pressures is placed in communication through a passage in the end plate with a zone of predetermined area associated with the end face of the end plate remote from the rotor and sealed from the surrounding area of that end face. In this way that sealed zone is subjected to fluctuating pressures which are substantially equal to but oppositely directed with respect to the fluctuating pressures to which the first said zone is subjected.

United States Patent [191 Eglington [451 Sept. 3, 1974 i 22 Filed:

John Christopher Eglington, Flint, Stroud, England [73] Assignee: Dowty Hydraulic Units Limited,

Cheltenham, England Mar. 21, 1973 [21] Appl. No.: 343,456

[75] Inventor:

[52] US. Cl. 418/132 [51] int. Cl...... F0lc 19/08, F030 3/00, F04c 15/00 [58] Field of Search 418/131, 132

[56] References Cited UNITED STATES PATENTS 2,809,592 10/1957 Miller et a1, 418/132 3,057,303 10/1962 Lauck........ 418/132 3,104,616 9/1963 Peet 418/132 3,270,680 9/1966 Rich 418/132 3,292,551 12/1966 Gordon 1 418/132 3,490,382 1/1970 Joyner 418/132 3,539,282 11/1970 Forschner 418/132 Primary ExaminerC. J. Husar Assistant Examiner-John J. Vrablik Attorney, Agent, or FirmYoung and Thompson [5 7 ABSTRACT A positive-displacement machine having meshing rotors is provided with an end plate in the form of a joumal/thrust bush, associated with a rotor, which has a groove forming a speed slot associated with its face adjacent to the rotor. The end plate is so subjected to liquid pressures on defined areas of its end face remote from the rotor as to be urged into adequate sealing engagement with the rotor. A zone of the said face of the end plate adjacent to the rotor which is subjected, during operation of the machine, to fluctuating pressures is placed in communication through a passage in the end plate with a zone of predetermined area associated with the end face of the end plate remote from the rotor and sealed from the surrounding area of that end face. In this way that sealed zone is subjected to fluctuating pressures which are substantially equal to but oppositely directed with respect to the fluctuating pressures to which the first said zone is subjected.

6 Claims, 10 Drawing Figures PAIE-NIEDSEP 31914 3%3319' SHEET 10F 3 PAIENTEUSEP M 3.883319 sum 2 or a is ew W 3000 f 2 f 0 l l 2000 D (I) .53 1000 a 0;

A B c 0 E FA 8 c ROTATION POSITION (1.REVOLUT/0N) L-"R/SE POINT" HG 5 PAIENTEDSEP 319M v 3.8%.319 sum 30? 3 POSITIVE-DISPLACEMENT LIQUID-PRESSURE MACHINES AND PRESSURE-BALANCED JOURNAL/TI-IRUST BUSHES THEREFOR This invention relates to positive-displacement machines having meshing rotors (for example, gears) and having an end plate, associated with a rotor, which is pressure-balanced.

In such pressure-balancing, the face of the end plate remote from its rotor is subjected upon defined areas thereof to liquid pressures which act in opposition to the pressures applied to the face of the end plate adjacent to the rotor by the liquid being guided through the machine by the rotors, so that the end plate is urged into adequate sealing engagement with the side face of the rotor without the generation of undue friction between the end plate and the side face. The end plate may be a bushing means for the rotor and may be separate from, or joined to, an adjacent end plate of an adjacent rotor. A groove is provided in, or in association with, the face of the end plate adjacent to its rotor, this groove being in communication with the high pressure side of the machine so as to receive liquid therefrom, and being positioned so to distribute this liquid upon this face as to ensure that the positions of the centres of pressure of the liquid pressures applied to this face are substantially non-varying during operation of the machine.

The passage of high pressure liquid to this groove has either been direct from the high pressure side of the machine, or, alternatively, it has been by way of restrictor means.

A machine of the above kind is hereinafter referred to as a positive-displacement machine of the kind described.

Although hitherto in a positive-displacement machine of the kind described the provision of the groove has resulted in a substantial reduction in the tilting of the end plate in relation to the rotational axis of the respective rotor, a small amount of tilting has nevertheless occurred due to fluctuating pressures created in a zone of the rotor-engaging face of the end plate where the teeth of the respective rotor successively approach that end of said groove remote from the high pressure side of the machine, because these fluctuating pressures are in no way balanced axially of the end plate.

According to this invention there is provided a positive-displacement machine of the kind described in which a zone of the rotor-engaging face of the end plate which is subjected, during operation of the machine, to fluctuating pressures is placed in communication through a passage in the end plate with a zone of predetermined area upon or associated with the end face of the end plate remote from the rotor and sealed from the surrounding area of that end face, whereby that sealed zone is subjected to fluctuating pressures which are substantially equal to but oppositely-directed with respect to the fluctuating pressures to which the first said zone is subjected.

The said sealed zone may be defined by a recess formed in said remote end face or, alternatively, by a recess formed in that face of the casing of the machine adjacent to said remote end face, each such recesse being provided with a sealing ring around its periphery.

In each case the recess is preferably positioned within the confines of that one of said defined areas of the remote end face of the end plate which -is subjected to a low pressure, the sealing ring isolating said sealed zone from the area at low pressure which surrounds it.

Instead of providing a recess either in said remote face or in the casing of the machine, the said sealed zone may be defined by one end face of a piston slidable in a blind bore in that part of the casing of the machine adjacent to said remote end face, said piston projecting from the bore with its other end face engaging said remote end face and said passage communicating with said bore. In this way fluctuating pressures are applied to said one end face in opposition to the fluctuating pressures in the first said zone thereby urging the piston against said remote end face. The bore is preferably so positioned that the forces consequently applied by the piston to the end plate are directed within the confines of that one of said defined areas of said remote end face thereof which is subjected to a low pressure.

Since by the provision of the sealed zone the fluctuating pressures in the first said zone are substantially balanced axially of the end plate, the overall pressurebalancing of the end plate is more complete than had said passage and the said sealed zone not been provided.

Where the machine has two meshing motors with shafts extending from each side thereof, two of said end plates are provided for each rotor, such end plates being of substantially annular cross-sectional shape and rotatably supporting the shafts. The end plates may be arranged to fit with a degree of axial freedom in overlapping bores provided in the casing of the machine.

Each said groove may be formed in the outer circumferential edge of the respective end plate adjacent to the respective rotor.

The end plates for the shafts on one side or each side of the rotors may comprise separate members having external engaging flats, one end plate with respect to its adjacent end plate, or alternatively they may be integrally formed.

Two embodiments of the invention will now be particularly described by way of example with reference to the accompanying diagrammatic drawings, of which,

FIG. 1 is a cross-section of a positive-displacement machine in the form of a gear pump in accordance with the first embodiment,

FIG. 2 is a partial cross-section taken along the line II-II on FIG. 1,

FIG. 3 is a partial cross-section taken along the line IIIIII on FIG. 1,

FIG. 4 is an enlarged view of a part of FIG. 2 which shows one of the rotors of the machine in one rotational position,

FIG. 5 is a graphical representation of the pressure distribution on a part of the construction shown in FIG. 4,

FIG. 6 is a view similar to'that of FIG. 4 but showing the said rotor in another rotational position,

FIG. 7 is a view similar to that of FIG. 4 of a rotor and end plate of a machine in accordance with the second embodiment of the invention,

FIG. 8 is a view similar to that of FIG, 7 showing the rotor of the second embodiment in another rotational position,

FIG. 9 is an end view of the end plate shown in FIGS. 7 and 8 taken in a direction towards the rotor, and,

FIG. is a cross-section taken along the line X-X on FIG. 9.

With reference to FIGS. 1 to 6 of the drawings, a positive-displacement machine in the form of a gear pump 10 includes a casing 11 having a low pressure or inlet port, not shown in FIG. 1, but indicated diagrammatically in FIG. 2 at 12. The casing also has a high pressure or outlet port, also not shown in FIG. 1 but indicated diagrammatically at 13 in FIG. 2. The casing 1 1 houses two meshing pump rotors in the form of gears 14 and 15, the shafts 16, 17; 18, 19 of which are mounted for rotation in respective pairs of end plates in the form of journal thrust bushes 20, 21; 22, 23. The bushes are of D-shaped cross-section with flats of adjacent bushes in abutment as at 24 and 25 respectively. The bushes fit with a degree of axial freedom in suitable overlapping bores provided in the casing.

The shaft 19 passes through an aperture 26 in a pump closure member 27 which is bolted to the casing 11. Externally of the pump the shaft 19 is connectible to a power source (not shown).

The portions of the bushes 20 and 22 remote from the gears 14 and are provided with spigot-like projections 28 and 29 of pre-determined shape, around each of which a sealing ring 30, 31 is provided as shown in FIG. 1. The shape of these projections is clearly shown in FIG. 3, but in that figure the sealing rings are omitted. The gears 14, 15, rotate about axes 32, 33.

The defined areas of the bushes and 22 outside of the sealing rings 30 and 31 are subjected to high pressure from the outlet port 13 of the pump which gains access thereto through suitable passage means (not shown). The defined areas of the projections 28 and 29 within the sealing rings 30 and 31 are low pressure areas, the effective centres of area thereof being offset from the axes 32, 33 as necessary to contribute, in known manner, to pressure-balancing of the bushes 20 and 22 whereby to oppose the tendency of the bushes to tilt in operation, thus to reduce bush wear. The bushes 21 and 23 are similarly provided, and the chambers in the casing 11 subjected to high pressure are shown in FIG. 1 at 34 and 35, while the chambers subjected to low pressure are shown at 36 and 37.

In such pressure-balancing the liquid pressures upon the above-defined areas act in opposition to the pressures applied to the end faces 38 of the bushes adjacent to the gears by the liquid being carried through the pump by the gears, and ensure that the bushes are urged into adequate sealing engagement with the side faces of the gears without the generation of undue friction between the bushes and the side faces.

Grooves in the form of what are termed speed slots, whose precise function is later described, are provided, as at 39, around the circumferential edge of each bush adjacent to the gears in the position shown in FIGS. 2, 4 and 6, each speed slot extending through an angle of 75. The end of each speed slot closer to the low pressure port 12 is spaced 27 from the plane containing the axes 32 and 33.

As shown in FIG. 1, the gear shafts 16, l7, l8 and 19 are provided each with undercuts or relief recesses 40, 41, 42 and 43 immediately adjacent to the side faces of the respective gears.

The bushes 21 and 23 are similar, but of opposite hand, to the bushes 20 and 22 and also have speed slots as at 39.

Recesses 46 and 47 are provided adjacent to the low pressure port 12 in the faces 38 of the bushes, those of the bushes 20 and 22 being shown in FIG. 2. Holes 48 are drilled, parallel to the axes 32, 33, completely through the bushes respectively to communicate with the low pressure chambers 36, 37. Longitudinal grooves 50 are provided, one in each of the bores of the bushes 20, 21, 22 and 23, extending for the full length of the bores and placing the low pressure chambers 36 and 37 in communication with the relief recesses 40, 41, 42 and 43.

The faces 38 of the bushes are each provided with a shallow slot 51 which aligns with the slot 51 of its mating bush as shown in FIG. 2, and which opens to the relief recess 40, 41, 42 or 43.

The recesses 46, 47, holes 48, grooves 50, relief recesses 40, 41, 42, 43 and slots 51 are provided for low pressure lubrication of the shafts 16, 17, 18 and 19 as they run in their bushes in the manner disclosed in the specification of US. Pat. No. 3,490,382.

Each bush is provided with a passage in the form of another hole 52, parallel with the hole 48, extending from the face 38 of the bush adjacent to its rotor to the end face thereof remote from the rotor. Each hole 52 is placed somewhat less than mid-way between the hole 48 and the end of the slot 39 remote from the high pressure side of the pump.

The end of each hole 52 remote from the rotor opens into an elongated recess 53 formed in the face of that part of the pump casing 11 adjacent to the face of the respective bush remote from the respective rotor. This recess has radiussed end portions and a sealing ring 53a is provided, in association with the periphery of this recess, being sandwiched between the bush and the adjacent part of the casing. In this way the recess forms a zone sealed with respect to the surrounding area of said remote face.

In operation of the gear pump above described, with rotation of the driving shaft 19 the two intermeshing gears 14 and 15 rotate, drawing liquid through the low pressure port 12 and discharging this under high pressure through the port 13.

Liquid under high pressure is applied to the faces of the bushes 20, 21, 22 and 23 remote from the gears 14 and 15 over the areas 54 outside of the sealing rings 30, 31 while the areas 55 of these bushes within the sealing rings are subjected to liquid at low pressure. The sealing ring 53a associated with each recess 53 isolates the recess from the respective area 55 at low pressure which surrounds it.

The pressure distribution on the faces 38 of the bushes 20, 21, 22 and 23 adjacent to the gears is dependent upon the positions of the speed slots 39 because the latter determine the positions of the centres of pressure of the liquid pressures applied to those faces and thus the effective boundaries between the low pressure and the high pressure zones thereon. It will be understood that, as each gear, for example the gear 14 in FIG. 4, rotates, the inter-tooth spaces 56 successively start to move away from the low pressure port 12 and each then contains liquid at low pressure, Since each such inter-tooth space 56 is exposed to adjacent portions of the faces 38 of a pair of bushes, this low pressure is applied to these portions. In FIG. 4 positions A, B, C, D, E and F are marked around the bush corre sponding to rotational positions of the teeth 57 of the gear 14. FIG. 5 graphically shows in relation to those positions on FIG. 4 the pressure characteristics of the pump, it being seen that after a tooth 57 has passed the position B the pressure in the corresponding inter-tooth space rises steeply to full delivery pressure and is maintained there until that tooth reaches the position F, whereupon the pressure falls off very steeply. Thus, in this embodiment what is termed the rise point between low pressure and high pressure in successive tooth spaces occurs between positions B and C. But for the provision of the speed slots, such a rise point would be inconsistent and the said positions of the centres of pressure of the liquid pressures applied to the faces 38 of the bushes would vary in accordance with variation in delivery pressure, liquid viscosity and rotational speed of the pump. Such inconsistency would result, during operation of the pump, in a wide variation of the effective boundaries between the areas at low pressure and the areas at high pressure on the faces 38. Since the positions of the centres of pressure upon the said defined areas 54 and 55 do not vary, this inconsistency would thus give rise to imperfect balance of the bushes.

The speed slots 39 have the effect of reducing this imperfection by allowing some high pressure liquid at the high pressure port to pass back along them whereby it is so distributed on the faces 38 as to modify the pressure-balancing of the bushes. In consequence the said rise point is maintained substantially consistent, with the said positions of the centres of pressure of the liquid pressures applied to the faces 38 being substantially non-varying, this regardless of delivery pressure, liquid viscosity or rotational speed. In such pressurebalancing the inter-tooth spaces, as at 56, containing liquid at high pressure afford high pressure loading of the faces 38 in the desired zone in opposition to the high pressure on the areas 54, and the inter-tooth spaces containing liquid at low pressure afford low pressure loading of the faces 38 in the desired zone in opposition to the low pressure on the areas 55. This pressure-balancing is such that the bushes are urged into adequate sealing engagement with the side faces of the gears without the generation of undue friction and at the same time the fact that the modification to the balancing is afforded by the provision of the speed slots results in a substantial reduction, during operation, of tilting of the bushes in relation to the rotational axes of the gears. The high pressure areas on the faces are shown by shading at 58 and 59 in FIG. 2.

The fact that the speed slots 39 afford such reduction in imperfection in the pressure-balancing of the bushes irrespective of the viscosity of the liquid, the pressure of the liquid or the speed of rotation of the pump, results in the pump having substantially consistent performance characteristics across wide speed, pressure and temperature ranges.

The speed slots 39 do not directly communicate with the high pressure port. As shown in an exaggerated manner in FIGS. 4 and 6, during operation of the pump the high pressure on the outlet side of the pump loads the meshing gears transversely towards the inlet side thereof so that clearances are formed as at 60 in FIGS. 4 and 6 between the tips of those gear teeth at the high pressure side of the casing. This clearance, which is of the order of 0.005 inches, forms restrictor means through which liquid, indicated by the dotted arrows 61, from the high pressure port can be fed into the slots 39. Hence liquid entering the slots does so at reduced velocity and likewise leaves the slots at their ends closer to the low pressure port at reduced velocity also.

FIG. 4 shows a tooth 57 of the gear 14 just approaching the slot 39, while FIG. 6 shows the same tooth just opening the slot to the inter-tooth space 56 following that tooth. It is at this stage that liquid passes from the slot 39 into the inter-tooth space 56 which is at a lower pressure than the inter-tooth space in advance of that tooth 57. Thus the liquid, indicated by the dotted arrow 62, entering this inter-tooth space at lower pressure, does so at some velocity because it is passing from a higher pressure zone to a lower pressure zone. However, by virtue of the restrictor means 60 the pressure in the speed slot 39 is not so high as would cause this velocity to be as high as would be the case without the restrictor means. Hence, the liquid passing at such reduced velocity into the inter-tooth space at low pressure causes substantially no undesirable erosion of the material of the surfaces upon which this liquid impinges in the vicinity of the end of the slot 39 closer to the low pressure port.

With reference to FIG. 4 of the drawings and with the gear 14 rotating in the clockwise direction, as indicated when the tooth 57 is in the position drawn the intertooth space in advance thereof is, due to its communication through the speed slot 39 and the restrictor means 60 with the port 13, at pump delivery pressure or at a pressure closely approaching this. The intertooth space 56 on the trailing side of the tooth 57 is at some intermediate pressure between the pump delivery pressure and inlet pressure, while the inter-tooth space 63 on the low pressure side of the tooth 64 is open to inlet pressure because it is in communication through the recess 46 with the inlet port 12.

As the gear 14 moves further in the clockwise direction to the position where, as shown in FIG. 6, the tooth 57 opens the speed slot 39 to the inter-tooth space 56, high pressure derived from the port 13 is now present in that inter-tooth space. Further, the hole 52 is now closed from the intermediate pressure and is just open to the inter-tooth space 63. It will be understood therefore that during operation of the pump as each successive tooth approaches the end of the speed slot 39 remote from the high pressure port 13 as shown in FIG. 4, the hole 52 is subjected to the intermediate pressure prevailing on the respective face 38 between successive spaces 56 and 63, but as each successive tooth passes to the position shown by the tooth 57 in FIG. 6, the respective inter-tooth space 56 is subjected to high pressure and the hole 52 is opened to the respective intertooth space 63. Since that space is in communication with the inlet 12 through the recess 46, the hole 52 is subjected to inlet pressure, or approximately thereto.

Thus fluctuating pressures are continually occurring on the face 38 of the bush adjacent to the rotor and in the vicinity of the hole 52. t is very desirable that these fluctuating pressures should also be applied to the face of the bush remote from the rotor in order that they are balanced out axially of the bush, thus to achieve more perfect pressure-balancing of the bush. Accordingly the respective hole 52 opens into the elongated recess 53 so that the fluctuating pressures gain access to the recess. This recess is of such predetermined area and is so positioned within the low pressure area 55, being sealed with respect thereto, that the fluctuating pressures occurring on the face 38 in the zone of the hole 52 are sufficiently accurately balanced axially of the bush as to give a significantly improved overall pressure-balancing of the bush whereby desired reduction of bush wear is achieved.

Such improvement in pressure-balancing of the bushes is afforded at no detriment to the effect of the provision of the speed slots which maintain the said rise point substantially constant irrespective of the speed of the pump, the delivery pressure thereof, or the viscosity of the oil.

With reference now to FIGS. ,7 to of the drawings, there is shown an alternative embodiment of the invention. In this embodiment the gears 114 have ten teeth and differ also from the construction described with reference to FIGS. 1 to 6 in that instead of providing a recess in that face of the casing adjacent to the respective bush, with which recess a hole communicates for transmitting to the recess the fluctuating pressures referred to in the description of the first embodiment, each bush 120 has a hole 121 opening from a radiallydirected recess 122 in the end face 123 of the bush adjacent to the gear 114. Each hole 121 communicates with a respective hole 124 formed in an associated piston 125 carried in a blind bore 126 formed in the casing portion 127. This piston is of plastics material and thus is self-sealing in the bore.

A coil spring 128 is provided to the right in FIG. 10 of its piston 125 to hold the piston in initial and substantial sealing contact with the adjacent face of the respective bush 120. Thus the annular face 129 of the piston which is exposed to the chamber 130 to the right of the piston defines a sealed zone.

Since the holes 121, 124, place the zone 122 at which the said fluctuating pressures occur in communication with said sealed zone, the sealed zone is subjected to fluctuating pressures which are substantially equal tov but oppositely directed with respect to those at the zone 122. Thus the piston 125 which is initially urged at its face 131 against the bush by the spring 128, is also urged hydraulically by the fluctuating pressures in the chamber 130 against the bush. Each sealed zone is of such predetermined area and so positioned that the fluctuating pressures occurring at the zone 122 are sufficiently accurately balanced axially of the respective bush as to give significantly improved overall pressure balancing of the bush.

FIG. 7 shows the gear 114 in a position in which the inter-tooth space 132 in advance of the tooth 133 is subjected to high liquid pressure through the speed slot 134, while the inter-tooth space 135 in advance of the tooth 136 is subjected to an intermediate pressure. As shown, in this condition of the gear 114 the hole 121 is partly open to the inter-tooth space 135. In this embodiment the speed slot 134 is provided with a short extension groove 137 which is directed inwardly in the face 123 of the bush towards the respective gear shaft.

FIG. 8 shows the gear 114 in a position in which the teeth have progressed in the clockwise direction, such that the tooth 133 is passing over the extension groove 137. Under this condition liquid under high pressure in the speed slot 134 gains access to the inter-tooth space 135, but since to achieve this it passes between the respective end face of the tooth 133 and the face 123 of the bush, a pressure drop occurs. However, the pressure now prevailing in the inter-tooth space 135 is considerably higher than the pressure which was prevailing there with the gear 114 in the position shown in FIG. 7. In FIG. 8 the hole 121 is no longer open to the intertooth space but is instead now open to the space in advance of the tooth 138. As shown there is a slight lap-over between the tooth 138 and the upper edge of the recess 146 so that the pressure in the space in advance of the tooth 138 is only a little above the pressure prevailing at the inlet port 112.

The construction of the embodiment described with reference to FIGS. 7 to 10 therefore achieves the same objects as the construction described with reference to FIGS. 1 to 6.

I claim:

1. A positive-displacement liquid-pressure machine comprising a casing, having a low pressure port and a high pressure port, journal/thrust bushes supported for axial sliding movement within the casing, at least two meshing gears having shafts by which they are supported for rotation in said bushes, sealing means provided on the end face of each journal/thrust bush remote from its respective gear which sealingly divide that face into a first area, having an effective centre offset from the rotational axis of the respective gear and being subjectable to low liquid prssure derived from said low pressure port, and a second area subjectable to high liquid pressure derived from said high pressure port, whereby the bush is axially-urged into sealing engagement with the respective side face of said gear, groove-defining means in the end face of each bush adjacent to its gear, being in communication with said high pressure port and extending across that face in the direction towards the low pressure port so that the groove has an end which is disposed substantially closer to said low pressure port than to said high pressure port, whereby during operation of the machine a zone of that face, which is disposed at a position between said end of the groove and the low pressure port, is subjected, due to the teeth of the respective gear successively passing that end of the groove, to fluctuating liquid pressures, and passage-defining means formed in the bush and extending therethrough to place said zone in communication with another zone, of predetermined area, associated with said first area and generally in alignment through that first area with the firstmentioned zone, and said other zone being sealed by a sealing device from said first area, whereby during operation of the machine that sealed zone is subjected by way of said passage-defining means to fluctuating liquid pressures which are substantially equal to but oppositely-directed with respect to the fluctuating liquid pressures pertaining in the first-mentioned zone.

2. A machine as claimed in claim 1 wherein said sealed zone is defined by a recess formed in that face of the casing of the machine adjacent to said remote end face.

3. A machine as claimed in claim 2, wherein the recess is provided with a sealing ring, forming said sealing device, around its periphery.

4. A machine as claimed in claim 1, wherein said sealed zone is defined by one end face of a piston, said piston also forming the sealing device and being slidable in sealing relation in a blind bore in that part of said casing adjacent to said remote end face, said piston projecting from the bore with its other end face engaging said remote end face and the passage communicating with said bore.

5. A machine as claimed in claim 4, wherein the bore is so positioned that the forces consequently applied by the piston to the bush are directed within the confines of said first area of said remote end face of that bush.

6. A positive-displacement liquid-pressure machine comprising a casing, having a low pressure port and a high pressure port, joumal/thrust bushes supported for axial sliding movement within the casing, at least two meshing gears having shafts by which they are supported for rotation in said bushes, sealing means provided on the end face of each journal/thrust bush remote from its respective gear which sealingly divide that face into a first area, having an effective centre offset from the rotational axis of the respective gear and being subjectable to low liquid pressure derived from said low pressure port, and a second area subjectable to high liquid pressure derived from said high pressure port, whereby the bush is axially-urged into sealing engagement with the respective side face of said gear, groove-defining means in the end face of each bush adjacent to its gear, being in communication with said high pressure port and extending across that face in the direction towards the low pressure port so that the groove has an end which is disposed substantially closer to said low pressure port than to said high pressure port, whereby during operation of the machine a zone of that face, which is disposed at a position between said end of the groove and the low pressure port, is subjected, due to the teeth of the respective gear successively passing that end of the groove, to fluctuating liquid pressures, and said casing housing a piston of predetermined cross-sectional area slidable in sealing relation in a blind bore in that part of the casing adjacent to said remote end face, said piston projecting from the bore and engaging said remote end face at a position generally in alignment through said first area with said first-mentioned zone, the face of the piston remote from the bush forming a zone which is sealed with respect to said bore and said first area, and passagedefining means axially-extending through the bush and the piston which place the first-mentioned zone in communication with the sealed zone, whereby during operation of the machine that sealed zone is subjected to fluctuating liquid pressures which are substantially equal to but oppositely-directed with respect to the fluctuating liquid pressures pertaining in the firstmentioned zone, and forces consequently applied by way of the piston to the bush being directed thereupon within the confines of said first area thereof. 

1. A positive-displacement liquid-pressure machine comprising a casing, having a low pressure port and a high pressure port, journal/thrust bushes supported for axial sliding movement within the casing, at least two meshing gears having shafts by which they are supported for rotation in said bushes, sealing means provided on the end face of each journal/thrust bush remote from its respective gear which sealingly divide that face into a first area, having an effective centre offset from the rotational axis of the respective gear and being subjectable to low liquid prssure derived from said low pressure port, and a second area subjectable to high liquid pressure derived from said high pressure port, whereby the bush is axially-urged into sealing engagement with the respective side face of said gear, groovedefining means in the end face of each bush adjacent to its gear, being in communication with said high pressure port and extending across that face in the direction towards the low pressure port so that the groove has an end which is disposed substantially closer to said low pressure port than to said high pressure port, whereby during operation of the machine a zone of that face, which is disposed at a position between said end of the groove and the low pressure port, is subjected, due to the teeth of the respective gear successively passing that end of the groove, to fluctuating liquid pressures, and passage-defining means formed in the bush and extending therethrough to place said zone in communication with another zone, of predetermined area, associated with said first area and generally in alignment through that first area with the first-mentioned zone, and said other zone being sealed by a sealing device from said first area, wherEby during operation of the machine that sealed zone is subjected by way of said passage-defining means to fluctuating liquid pressures which are substantially equal to but oppositelydirected with respect to the fluctuating liquid pressures pertaining in the first-mentioned zone.
 2. A machine as claimed in claim 1 wherein said sealed zone is defined by a recess formed in that face of the casing of the machine adjacent to said remote end face.
 3. A machine as claimed in claim 2, wherein the recess is provided with a sealing ring, forming said sealing device, around its periphery.
 4. A machine as claimed in claim 1, wherein said sealed zone is defined by one end face of a piston, said piston also forming the sealing device and being slidable in sealing relation in a blind bore in that part of said casing adjacent to said remote end face, said piston projecting from the bore with its other end face engaging said remote end face and the passage communicating with said bore.
 5. A machine as claimed in claim 4, wherein the bore is so positioned that the forces consequently applied by the piston to the bush are directed within the confines of said first area of said remote end face of that bush.
 6. A positive-displacement liquid-pressure machine comprising a casing, having a low pressure port and a high pressure port, journal/thrust bushes supported for axial sliding movement within the casing, at least two meshing gears having shafts by which they are supported for rotation in said bushes, sealing means provided on the end face of each journal/thrust bush remote from its respective gear which sealingly divide that face into a first area, having an effective centre offset from the rotational axis of the respective gear and being subjectable to low liquid pressure derived from said low pressure port, and a second area subjectable to high liquid pressure derived from said high pressure port, whereby the bush is axially-urged into sealing engagement with the respective side face of said gear, groove-defining means in the end face of each bush adjacent to its gear, being in communication with said high pressure port and extending across that face in the direction towards the low pressure port so that the groove has an end which is disposed substantially closer to said low pressure port than to said high pressure port, whereby during operation of the machine a zone of that face, which is disposed at a position between said end of the groove and the low pressure port, is subjected, due to the teeth of the respective gear successively passing that end of the groove, to fluctuating liquid pressures, and said casing housing a piston of predetermined cross-sectional area slidable in sealing relation in a blind bore in that part of the casing adjacent to said remote end face, said piston projecting from the bore and engaging said remote end face at a position generally in alignment through said first area with said first-mentioned zone, the face of the piston remote from the bush forming a zone which is sealed with respect to said bore and said first area, and passage-defining means axially-extending through the bush and the piston which place the first-mentioned zone in communication with the sealed zone, whereby during operation of the machine that sealed zone is subjected to fluctuating liquid pressures which are substantially equal to but oppositely-directed with respect to the fluctuating liquid pressures pertaining in the first-mentioned zone, and forces consequently applied by way of the piston to the bush being directed thereupon within the confines of said first area thereof. 